This invention relates, in general, to an apparatus, method and system arranged to minimise power consumption in a wireline environment, and is especially (but not exclusively) applicable to a wireline communication system that communicates relatively high frequency broadband signals over relatively low frequency voiceband signals, e.g. by using digital subscriber line (xDSL) communication protocols in bi-directional twisted-pair systems.
Telecommunication systems that interconnect wireline subscriber terminals are being developed to support broadband data communication More particularly, recent developments in broadband communication protocols allow broadband data to be overlaid on narrowband voice or integrated service digital network (ISDN) traffic. Specifically, the interconnection of broadband modems located at the subscriber terminal and at an exchange allow current broadband access systems to communicate on spare spectrum (i.e. spare frequency channels) of a twisted pair communication resource; the spare frequency channels being isolated from conventionally encoded voice signals by a suitable filter. In this respect, and depending upon the complexity of the xDSL coding scheme, overlaid broadband systems can support data rates in excess of two Megabits per second (Mbps), although this rate is dependent upon the physical parameters of the connection, e.g. the overall length of the twisted pair and its composition and configuration.
Asymmetric Digital Subscriber Line (ADSL) and High-speed Digital Subscriber Line (HDSL) protocols, for example, can support data rates of 2 Mbps over distances of approximately three kilometres, while more complex schemes (Such as VDSL) can support data rates of 8 Mbps and above over distances of typically, less than two kilometres. Protocols such as Very high-speed Digital Subscriber Line (VDSL) utilise multiple sub-channel carriers, e.g. in a discrete multi-tone (DMT) environment, to provide an adaptive system that mitigates the effects of cross-talk by selectively ignoring noise-effected sub-channel carriers or reducing the number of bits supported on each sub-channel. As will be appreciated, DMT provides a comb of frequency carriers that are each separated modulated and then combined to generate a composite signal envelope. As such, information (both control information and traffic) is distributed across a number of different frequency carriers.
DMT schemes for supporting, for example, VDSL are often realised in a time division duplex (TDD) transmission environment in which a single communication resource, i.e a frequency band, supports both up-link and down-link transmissions using the same frequencies. In other words, there is a sharing in time of the bandwidth provided by the extended spectrum. The use of guard periods between adjacent groups of time-slots within a TDD frame ensures that rogue overlapping transmissions within the up-link and down-link do not occur, and hence eliminates the likelihood of near-end cross talk (NEXT). In more detail, the guard periods provide a period in which a power amplilier can power-up and power-down, and also allow for some adjustment (i.e. alignment) of the frame with respect to a selected pilot tone on a designated sub-channel carrier of a DMT scheme.
In relation to bundles of wireline communication resources, it is also important to understand the potentially undesirable effects associated with cross-talk interference. Specifically, with bi-directional communication, the relative location of the lines, for example, between twisted copper-pair causes cross-talk interference to be induced into proximately located wireline communication resources (principally by the mechanisms of capacitive and inductive coupling and by radiation arising from the imperfect nature and performance of the cabling). Moreover, where symmetrical and asymmetrical service are simultaneously required on pairs in the same bundle, cross-talk becomes a significant problem, as will readily be appreciated.
For completeness, it will be understood that near-end cross-talk (NEXT) occurs when electromagnetic interference is induced into a wireline resource that is communicating information in an opposing direction, e.g. down-link (or downstream) information appears as noise in an up-link (or upstream) path. NEXT is undesirable because near-end generated interference is at a level that can potentially swamp data signals received from a remote terminal, which data signals have previously been subjected to attenuation through the transmission path. Furthermore, NEXT increases significantly at the higher frequency components and so is even more undesirable in high frequency data-over-voice wireline systems, such as VDSL. To avoid the harmful effects of Near-End Cross-Talk (NEXT) in a TDD system, an ensemble of collated communication resources must have synchronised and aligned transmissions However, in a mixed symmetrical/asymmetrical system. NEXT often occurs where the two opposing schemes have either different frequency allocations (in frequency division duplex, FDD) or different time slot allocations (in TDD).
With regard to Far End Cross-Talk (FEXT), this form of cross-talk affects non-addressed ports of a remote terminal. In other words, FEXT occurs when electromagnetic interference (i.e. noise) is induced into a wireline resource that is communicating information in a similar direction, e.g. upstream (or up-link) information appears as noise in another upstream wireline resource to an extent that performance on a given pair is limited. The effects of FEXT are correspondingly reduced by the attenuation path of the wireline resource. However, when multiple separate modem links exist (as supported by a multiplicity of different copper pairs proximally located towards the exchange LTE as a bundle of pairs in the access network and then fanned out to individual drops serving particular CPEs), crosstalk between the numerous signals at (or towards) the exchange presently generates noise that limits data-rate performance of both a given pair and the entire wireline system, in general. In synchronised systems, FEXT is inherent.
FEXT on adjacent pairs can be severely exacerbated from increased signal strengths at a receiver modem of the exchange LTE for the pair causing the FEXT. More especially, where these adjacent pairs have shorter reaches (i.e. shorter cable lengths), the attenuation of the signal in the wireline resource from such relatively closely located modems (as opposed to remotely located modems) is relatively little and, correspondingly. FEXT induced into adjacent wireline resources can be relatively large. In other words, in instances when FEXT from a relatively closely located modem is introduced into a wireline resource serving a distantly located modem, the FEXT interference effects can be catastrophic and corruption of the data from the distant modem absolute. For this reason, it is accepted that there is a need to xe2x80x9cback-offxe2x80x9d the power transmitted by the transmitting (CPE) modem in the upstream direction for all but the longest lines. Unfortunately, the necessity for back-off results in inefficient utilisation of the spectrum as a consequence of CPEs served by short loop distances having to forego the benefits of better signal to noise ratios (SNR) and therefore to restrict channel throughput by reducing power and lowering the bit transmission rate. In summary, FEXT is particularly problematic in the up-link at the LTE and limits spectral capacity generally.
The term xe2x80x9cself-FEXTxe2x80x9d will be understood to mean FEXT arising from use of the same time-slot and/or the same frequency for a common form of service (as opposed to differing services on a common wireline resource, such as a combination of ADSL and VDSL).
In order to establish effective end-to-end communication in a communication system, it is necessary for synchronisation between a transmitting unit and an interconnected receiving unit to occur, this is true for both a radio frequency environment and a wireline environment, such a VDSL system employed over a twisted pair. More specifically, synchronisation is required to demodulate encoded signals that are addressed to the receiver. In this respect and in relation to a DMT system (or the like, such as an orthogonal frequency division multiplexed OFDM scheme), a pilot carrier or tonal is used in a training sequence on a dedicated (pre-allocated) sub-channel. Initially, upon receipt of the pilot tone, the receiver acquires frequency lock and then establishes phase lock.
Another concern in relation to the implementation of wide bandwidth applications (required multi-carrier schemes) arises from the fact that the frequency spectrum seldom provides a homogeneous transmission environment and usually exhibits significant variations in its signal to noise (S:N) ratio across its bandwidth. In fact, portions of the spectrum are often unusable as a consequence of the presence of interference, such as noise and intermodulation products. To avoid these poor spectral regions, modems associated with the transmitter and receiver negotiate for the best sub-channel carriers during training and reject those sub-channel carriers that have a performance below a predetermined and acceptable threshold.
Another significant problem in a multi-carrier environment pertains to the peak power of the composite envelope formed by a summation of the individual sub-channel carriers. While techniques have been developed to restrict these peaks, there is still a general requirement for DMT equipment to support a wide (large) dynamic range.
In relation to a typical (although simplified) model of a wireline transmission path, line termination equipment (LTE) typically has an interface for receiving user data, which user data is subsequently applied to a digital signal processor (DSP). As will readily be appreciated, the DSP is configured to perform many signal processing tasks, including DMT encoding.
Following DMT encoding of the data, resultant signals are converted into the analog domain (in a digital-to-analog converter, DAC) before being applied to a power amplifier that is required to ensure that there is sufficient power to drive the wireline communication resource and therefore to ensure reception of the signals at the addressed customer premise equipment (CPE). As will be understood, a passive filter is generally included in the path between the power amplifier and the wireline, with a line impedance of the wireline having a nominal impedance value of approximately 100xcexa9 (with the actual value being country specific). In other words, the power amplifier is required by virtue of the fact that copper pairs have low impedance and DAC operation is subject to restricted operational parameters.
In relation to the aforementioned transmission path, it will be appreciated that the functional elements are attributable to individual line cards of telecommunication exchanges, such as private branch exchanges (PBXs) and infrastructure nodes that integrate with ancillary services, e.g. Internet server.
Unfortunately, the power amplifier is responsible for dissipating approximately half of the total power of the line card and so its inclusion and operation generates significant heat (that must effectively be, channelled away from temperature sensitive integrated circuits) whilst also placing significant demands on power supplies within the exchange. Consequently, systems and schemes that reduce total power consumption in line cards are very desirable.
According to a first aspect of the present invention there is provided a wireline communication system comprising line termination equipment coupled to customer premise equipment through a wireline communication resource supporting a multi-carrier transmission scheme between the LTE and the CPE, the wireline communication system comprising means for indicating activity of the CPE; and a controller, responsive to the means for indicating activity, for selecting sub-channel carriers of the multi-carrier transmission scheme on which information is modulated for transmission to the CPE over the wireline resource, wherein the controller is arranged to limit a number of sub-channel carriers utilised by the LTE in response to the means for indicating activity identifying that the CPE is inactive.
The LTE typically comprises a line driver coupled to a voltage supply and arranged to amplify signals to be applied to the wireline communication resource, the voltage supply being operationally responsive to the controller and wherein the controller is arranged to reduce a voltage applied to power the line driver in response to the means for indicating activity identifying that the CPE is inactive.
In a preferred embodiment, the LTE comprises a signal processor operationally responsive to the controller, the controller arranged to at least partially disable operationally functionality of the signal processor in response to the means for indicating activity identifying that the CPE is inactive.
In another embodiment, the wireline communication system comprises a memory containing at least one pattern having relatively few sub-channel carriers in relation to a number of sub-channel carriers utilised during information transfer in a data path between the LTE and CPE, the LTE further comprising a switch operationally responsive to the controller and configured to couple one of the date path and the memory to the wireline communication resource, the controller further arranged to cause the at least one pattern to be applied to the wireline communication resource in response to the means for indicating activity identifying that the CPE is inactive.
The controller (28) maintains at least one sub-channel carrier in response to the means for indicating activity identifying that the CPE is inactive, this ensures that the CPE maintains synchronisation and reasonable lock.
In another aspect of the present invention there is provided a line card for coupling to customer premises equipment through a wireline communication resource supporting a multi-carrier transmission scheme, the line card comprising: means of establishing a presence of CPE activity; and a controller, responsive to the means of establishing a presence of CPE activity, for selecting sub-channel carriers of the multi-carrier transmission scheme on which information is modulated for transmission to the CPE over the wireline resource, wherein the controller is arranged to limit a number of sub-channel carriers utilised by the LTE in response to the means for indicating activity identifying that the CPE is inactive.
In yet another aspect of the present invention there is provided a method of reducing power consumption in line termination equipment coupled to customer premise equipment through a wireline communication resource supporting a multi-carrier transmission between the LTE and the CPE, the method comprising the steps of: determining activity of the CPE; and assigning a relatively large number of sub-channel carriers to support information modulation onto the wireline communication rescurce in a multi-carrier transmission; and utilising a reduced number of sub-channel carriers for transmissions from the LTE in response to the step of determining activity of the CPE identifying that the CPE is inactive.
In still yet another aspect of the present invention there is provided a method of establishing a multi-carrier transmission between a line card of an exchange and customer premise equipment through a dedicated wireline communication resource, the exchange coupled to a plurality of CPEs through a plurality of wireline resources in which cross-talk is experienced, the method comprising the steps of: at the CPE, monitoring a cross-talk environment to identify an uplink transmission slot to the LTE, transmitting a wake-up message to the CPE within the uplink transmission slot, at the LTE, receiving the wake-up message and sending to the CPE at least one pilot tone, and at the CPE, receiving the at least one pilot tone and acquiring synchronisation and lock therefrom.
Advantageously, therefore, the present invention reduces power consumption within a line card and line termination equipment (at an exchange, for example). Moreover, by virtue of the fact that the line driver or power amplifier can be backed-off, heat generation within the line card is reduced. Beneficially, with reduced power consumption and reduced heat generation, temperature regulation devices (such as heat sinks and fans) can be scaled down within exchanges which simplifies exchange construction and reduces manufacturing costs. Furthermore, with battery powered equipment (e.g. required when the mains supply is interrupted), prolonged operational life is obtained through lower power consumption.